Method for block-based digital image watermarking

ABSTRACT

A method of encoding a watermark into a digital image. The method includes partitioning an original image into blocks and then determining a variance value of each block. The blocks are then arranged according to the variance values. A number of selected blocks are identified, where the number depends upon the size of the watermark. The blocks selected are then arranged by their location. Data from the watermark is used to replace bits in a representation of each of the selected blocks with data from the watermark. A method of decoding is also discussed.

BACKGROUND

1. Field

This disclosure relates to digital image watermarking, more particularlyto a method for inserting a watermark into blocks of an image.

2. Background

Traditionally, a watermark is an image formed on the surface of paper.When held up to a light, the watermark image can be seen in the paper.One application of such marks is to verify the authenticity of documentsor other printed images. Currencies, money orders, legal documents,among many other examples, are typically printed on paper stock havingthe appropriate watermark. When these printed images are received, therecipient can hold the paper up to the light and verify itsauthenticity. Using specially ‘coded’ paper places an additional layerof security in the process of creating printed images.

With the advent of computer networks, including the Internet, manyimages are copied from providing sites and passed around without everbeing printed. It becomes problematic for image providers to trackproprietary images. A copy of a digital image will typically be theexact same as the original image, unless there was some corruption inthe data. Providers of these images have no way to detect if this personobtained this image in violation of intellectual property laws, or ifthe copy obtained belongs to the image provider. In addition, recipientsof a digital image may want to ensure that the digital image received isfrom a particular source and is not a hoax or fabricated image. Thewatermark can be extracted that provides this information.

In response to this problem, a new industry of digital watermarking hasarisen. A digital watermark is typically some sort of identifying imagethat is ‘hidden’ in the original image data. Digital images can havefrom hundreds to millions bits of data. A digital watermark is buriedsomewhere in the image data in such a manner that it can be extracted bythose who know how and where it was buried, yet remain undetected bythose that do not know the necessary information. In this way, illegalcopying of images can be detected upon analysis of the image, and theindividual performing the copying cannot detect and eliminate thewatermark. Similarly, verification of authentic images can be performedto ensure that images received are from a particular source.

However, inserting watermarks into digital images have two problems.While the ability to bury the data in complicated methods allows forhigher security, too much complication can increase processing timebefore the digital image data is resolved into an image. Alternatively,if the watermark is too simple, it may be removed by a simple filteringoperation, rendering the watermark useless. Further, watermarks candegrade the quality of the image, as manipulation of the image data mayresult in objectionable artifacts.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by reading the disclosure withreference to the drawings, wherein:

FIG. 1 shows a flowchart of one embodiment of a method for encoding awatermark into a digital image, in accordance with the invention.

FIG. 2 shows a block diagram of an original image being divided intoblocks, in accordance with the invention.

FIG. 3 shows a block diagram of a block used to partition an originalimage, in accordance with the invention.

FIG. 4 shows a flowchart of one embodiment of a method for decoding awatermarked digital image.

DETAILED DESCRIPTION OF THE EMBODIMENTS

One embodiment of a method for inserting a copyrighted image into adigital image is shown in FIG. 1. The image that will be watermarkedwill be referred to as the original image. The watermark is the imagethat is to be inserted into the original image. This watermark image issome sort of copyrighted image, such as a company logo. Once theinsertion has been completed, the original image that has been alteredby the watermark will be referred to as the watermarked image.

For purposes of this discussion an original image of 256 pixelshorizontal extent by 256 lines of vertical extent (256×256), without anyintention of limiting the scope of this invention. A gray scale image of8 bits per pixel will be assumed, but any number of bits per pixel maybe used. As will be seen further, the techniques of the invention mayalso be extended to color images, such as those having 8 bits per eachcolor for each color red, green and blue. Also for this example, a logoimage of 16 pixels by 16 lines (16×16) having 1 bit per pixel will beassumed.

Referring now to FIG. 1, the original image such as 24 shown in FIG. 2,will be partitioned into blocks at 10. For this example, a block 26 ofdimensions 6×6 will be used. At 12 the variances of all blocks isdetermined. For example, the number of samples in the block is 36, eacha number between 0 and 255, using the assumption of 8 bits per pixel. Inthe following formula, each of these numbers will be x_(i).$\sigma^{2} = \frac{{n{\sum\limits_{i = 1}^{n}\quad x_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{n}\quad x_{i}} \right)^{2}}{n\left( {n - 1} \right)}$

Once the variance of each block is determined, the blocks are arrangedin order by their variance values at 14. In this example, the blocks maybe arranged in ascending order by their respective variance values. Ablock is denoted by the location of its starting pixel (i,j). Only thenumber of blocks needed for hiding the logo image may be selected. Inthis example, only two bits of the logo image will be stored in eachblock so in this case 128 blocks are required.

The blocks to be selected are identified at 16. In one embodiment theblocks are selected such that the mean gray level values of the blocksare either less than T₁ or greater than T₂, where T₁ and T₂ are certaintwo specified threshold values. The value of T₁ should preferably beclose to ‘0’, or the most minimum possible gray level. T₂ should beclose to ‘255’, or the maximum possible gray level value. However, thedifference between T₁ and T₂ to ‘0’ and ‘255’ respectively, is relativeand is strongly image dependent. Users may choose the values of T₁ andT₂ by checking the degradation in the image quality effected by theinsertion of the logo.

Once the appropriate blocks are identified and selected, they may bereordered by their positions in the original image, in ascending order.For purposes of this example, an (i,j)-th block is considered less thanthe (k,l)-th block if

(a) i<k, or

(b)j<l, if i=k.

Finally, a representation of each block is located and replaced withbits from the watermark image. In one example, the representation is themean value of each block truncated to the nearest integer. The least twosignificant bit in the mean value 8-bit representation are replaced bytwo bits chosen from the logo image, resulting in a changed mean value.Each pixel value of each of the selected blocks is replaced by thecorresponding changed mean value, making each block have zero variance.

The 16×16 watermark image can be thought of as a binary string of length256. The first 2 bits of the string is used to modify the mean of thefirst selected block. The next two bits are used to modify the nextchosen block and so on. This procedure may lead to minimal blockingartifacts in the watermarked image, though these may not be significantas the block size is very small compared to the size of the image. Onecan expect less distortion due to blocking, if the original image inwhich the watermark is to be hidden is of larger size.

The variance values of certain blocks that are not needed for hiding,may be too close to those of the selected blocks. This may lead tosharply delineated degradation areas in the image. To ensure robustness,variances of such blocks are adjusted at 22 by an adaptive enhancementprocedure described below:

For each such block, let

(i) mean=bl_mean (say)

(ii) minimum gray level value=bl_min (say)

(iii) maximum gray level value=bl_max (say)

The following relationships can be determined

(i) bl_max−bl_mean=high difference value;

(ii) bl_mean−bl_min=low difference value;

(iii) new_max=bl_max+high difference value;

(iv) new_min=bl_min−low difference value.

Replace each pixel value of the block with an adjusted pixel value basedupon these relationships. For example, an adjusted pixel value could bedetermined by:${\left\lbrack \frac{\left( {{new\_ max} - {new\_ min}} \right)}{\left( {{bl\_ max} - {bl\_ min}} \right)} \right\rbrack*\left( {{gray\_ value} - {bl\_ min}} \right)} + {{new\_ min}.}$

In this procedure, the mean of each block will remain unchanged, whereasthe variance of each block is increased resulting graceful degradationof the image.

As mentioned previously, the information used in encoding the image,including the size of the original image and the watermark image can beused to decode the image. Decoding the image is generally a reverse ofthe encoding procedure. An embodiment of a method for decoding an imageis shown in FIG. 4.

At 30, the watermarked image is partitioned, into non-overlapping squareblocks of size 6×6. The variance of each block is determined at 32. At34, the blocks are arranged in ascending order of variances. To extractthe hidden information, the decoder needs to correctly identify thoseblocks where the information is hidden. For this example, the encodingmethod shown in FIG. 1 is assumed. Therefore the information is hiddenin those blocks whose variance values are small. However due tonon-destructive attacks such as median or mean filtering, thevariability of those blocks may change to some extent. Further, it ishard to estimate the change in their variance values. The sorting of theblocks with respect to their variance values is needed.

The sorting of the blocks is followed by the choice of 128 blocks amongthem at 36. The number of blocks needed is determined by the size of thewatermark image, in this example a 16×16 image and two bits are used perblock. One example of how to select the blocks is to compute thevariance of four pixels that are at the middle of the block. Forexample, in a 6×6 block 24, shown in FIG. 2, pixel values at (3,3),(3,4), (4,3) and (4,4) positions are considered. It is expected that thepixel values of these four positions will be least affected bynon-destructive attacks such as median and mean filters. Those blocksfor which the newly computed variance values are close to zero are thenselected. Note that one needs to select the number of blocks in whichthe information is hidden, or 128 blocks for this example.

At 38, the center portion of each selected block is extracted bylocating a pixel lying at the middle of the block. For example, in the6×6 block of FIG. 3, any one of the pixel values that are either at(3,3), (3,4), (4,3), or (4,4) position is considered. The two leastsignificant bits of the selected pixel value will provide theinformation hidden in the block Once the center portion is extracted, inthis case 2 bits per block, the extracted portions need to be arranged.The location of the block gives the position of each set of 2 bits inthe logo image. All of the 128 2-bit samples are arranged in a 256-bitdata string that is then resolved into the 16×16 watermark image. Theability to extract this image confirms the authenticity or source of theoriginal image.

As a test, this method was coded in Matlab and run on a Sun Ultra-60workstation. It was observed that if the watermarked image is corruptedby a mean filter used twice, or by a median filter used thrice, recoveryof the logo can still be made. The watermarked image is found to bevisually indistinguishable from the original one. The quality of digitalimage watermarking in the methods discussed above will depend on therelative information content of the original image and the watermarkimage.

The methods and processes of the invention, including the examples setout above, will probably be implemented as software code included on acomputer readable medium such as a diskette, CD-ROM or downloadablefile. The software code will result in implementation of the methods ofthe invention when that code is executed.

Thus, although there has been described to this point a particularembodiment for a method and apparatus for block-based digital imagewatermarking, it is not intended that such specific references beconsidered as limitations upon the scope of this invention exceptin-so-far as set forth in the following claims.

What is claimed is:
 1. A method of encoding a watermark into a digitalimage, the method comprising: a) partitioning an original image intoblocks; b) determining a variance value of each block; c) arranging theblocks according to the variance values; d) identifying a number ofselected blocks, wherein the number depends upon the size of thewatermark; e) arranging the selected blocks by location; and f)replacing bits in a representation of each of the selected blocks withdata from the watermark.
 2. The method of claim 1, wherein arranging theblocks further comprises arranging the blocks in ascending order by thevariance values.
 3. The method of claim 1, wherein the identifying of anumber of selected blocks further comprises selecting blocks with a meangray level value between two thresholds.
 4. The method of claim 1,wherein the location of a block is determined by a location of a firstpixel in a block.
 5. The method of claim 4, wherein arranging theselected blocks by location further comprises arranging blocks with ahigher horizontal position ahead of blocks with a higher verticalposition.
 6. The method of claim 5, wherein vertical positions will beused to arrange blocks if horizontal positions are the same.
 7. Themethod of claim 1, wherein replacing bits in a representation of each ofthe selected blocks further comprises: a) determining a mean value ofeach selected block, wherein the mean value will be the representationof each block; b) truncating the mean value of each block to a nearestinteger value, producing a truncated mean value; c) replacing at leasttwo of the least significant bits of the truncated mean value with acorresponding number of bits from the watermark image producing achanged mean value; and d) replacing pixel values in each selected blockwith the changed mean value.
 8. The method of claim 1, wherein themethod further comprises: a) identifying unselected blocks havingvariance values near variance values of selected blocks; and b)replacing pixel values in each unselected block with an adjusted pixelvalue.
 9. A method for decoding a watermarked image, the methodcomprising: a) partitioning a watermarked image into non-overlappingblocks; b) determining a variance value for each block; c) arranging theblocks in order of the variance values; d) selecting a predeterminednumber of blocks, wherein the number depends upon the size of awatermark encoded in the watermarked image; e) identifying selectedblocks, wherein a center variance of each selected block isapproximately equal to zero; and f) extracting a portion of a centerpixel value for each selected block; and g) arranging the portionsextracted from each selected block by location of the selected blockinto a data string; and h) resolve the data string into the watermarkimage.
 10. The method of claim 9, wherein the blocks are arranged inascending order of the variance values.
 11. The method of claim 9,wherein the center variance is determined by computing the variance offour center pixels of each block.
 12. The method of claim 9, wherein thecenter pixel value is the value of a pixel at the middle of each block.13. The method of claim 9, wherein the portion extracted is comprised ofthe two least significant bits of the center pixel value.
 14. Acomputer-readable medium including software code that, when executed,results in: a) partition of an original image into blocks; b)determination of a variance value of each block; c) arrangement theblocks according to the variance values; d) identification of a numberof selected blocks, wherein the number depends upon the size of thewatermark; e) arrangement of the selected blocks by location; and f)replacement of bits in a representation of each of the selected blockswith data from the watermark.
 15. The medium of claim 14, wherein themedium also includes software code that when executed, results in: a)partition of a watermarked image into non-overlapping blocks; b)determination of a variance value for each block; c) arrangement of theblocks in order of the variance values; d) selection of a predeterminednumber of blocks, wherein the number depends upon the size of awatermark encoded in the watermarked image and the blocks selected havea center variance approximately equal to zero; e) extraction of aportion of a center pixel value for each selected block; f) arrangementof the portions extracted from each selected block by location of theselected block into a data string; and g) resolution of the data stringinto the watermark image.
 16. The medium of claim 14, wherein theoriginal image is 256 pixels by 256 lines, each pixel having 8 bits ofdata.
 17. The medium of claim 14, wherein blocks are 6 pixels by 6lines.
 18. The medium of claim 14, wherein the watermark image is 16pixels by 16 lines, each pixel having 1 bit of data.
 19. The medium ofclaim 14, wherein the portion extracted is comprised of two leastsignificant bits of data.